1. Field of the Invention
The invention relates to a method for printing a substrate with ink drops according to the “drop-on-demand” principle, which substrate is provided with a polymeric ink-receiving layer, using an ink jet printing device, the printhead of which is provided with a piezo element for generating ink drops, the method comprising the steps of supplying the substrate, generating ink drops and depositing the generated ink drops on the substrate, wherein the ink from which the ink drops are formed, has an ink composition which comprises a dye, water, a lower alcohol and humectant.
2. Description of the Related Art
Ink jet printing devices having its printhead provided with a piezo element for generating ink drops, are generally known in the specialist field. The piezo element can electrically generate a shock wave in the printhead, resulting in the formation of a drop from the ink in the printhead. In this way, it is possible to generate ink drops whenever they are required for printing the substrate, a technique also known as drop-on-demand (DOD) in the specialist field. The present DOD technique, which uses piezo elements, is also known as the piezo-DOD technique. An alternative to piezo-DOD within the drop-on-demand technique is a printhead with heating elements which are used to spray drops as a result of a vapour bubble being formed directly behind the jet nozzle with the aid of electrically controlled heat pulses in the nozzle channel filled with ink, the vapour bubble being used to launch a drop of ink. This alternative technique is also known as thermal-DOD and gives a printed result of which the combination of image quality and reproducibility is inferior to the printing result of piezo-DOD. Moreover, the service life of the heating element used in thermal-DOD is lower compared to the piezo element used in piezo-DOD.
Apart from the above, a substrate which is provided with a continuous, polymeric ink-receiving layer is in itself generally known in the specialist field and is also known as “resin coated paper” (RC paper). A substrate of this type can be printed with a high image quality, good reproducibility and high colour stability and consistency. An important application is the printing technique known as “proofing”. Examples of prints covered by the term proofing include colour guides, and catalogues or advertisements in which the colour reproduction has to be as accurate as possible, such as for example for furniture or clothing. Proofing therefore requires a high image quality, reproducibility and consistency/colour stability of the printed result. Conventional substrates of the microporous or open type are inadequate for use for proofing, since the printed result becomes discoloured too quickly, i.e. has a poor colour stability. This discoloration occurs, inter alia, on account of the fact that the structure has a large contact surface area with the surrounding atmosphere, via which chemical degradation of the dye occurs. Printed substrates with a polymeric ink-receiving layer do have a high colour stability, since the polymer swells as it receives the ink and binds the dye into the ink-receiving layer, and the ink-receiving layer has a much lower contact surface area with the surrounding atmosphere compared to microporous substrates.
For the above reasons, the combination of piezo-DOD with RC paper is advantageous, inter alia, for proofing printing, since good printed results can only be achieved currently by using more expensive techniques, such as offset techniques (in which a separate printing forme is required for each printed image) or “continuous ink jet” techniques (which require more expensive equipment than the drop-on-demand technique).
Piezo-DOD requires inks having a viscosity of at least 3 centipoise (cP), whereas for thermal-DOD a lower viscosity of ink can be used. In the case of piezo-DOD, the viscosity of the ink needs to be at least equal to this level, since a shock wave by which an ink drop is generated needs to be extinguished or damped by the mass of ink which remains in the printhead, before a next shock wave can be applied for generation of a next ink drop. The higher the viscosity of the ink, the more quickly it absorbs the shock wave. In known inks for piezo-DOD, this viscosity is generally achieved by the addition of so-called humectants to the ink composition. Additionally, humectants have the function of preventing the inks from drying out both during printing, especially at the nozzle of the printhead, as well as during storage of the inks. Humectants generally have high hydroscopic values. At the same time, humectants have a low vapour pressure, with the result that when substrates with a polymeric ink-receiving layer are used for printing by means of piezo-DOD, the ink drops applied are very slow to evaporate. This results in mixing effects on the paper, which have an adverse effect on the quality of the printing result. In addition, the printed substrate remains sticky for a long time after it has been printed. These adverse effects are, logically, further enhanced when the content of humectants is increased in order to achieve the viscosity required for piezo-DOD.
It is an object of the present invention to entirely or partially solve at least one of the above problems.